Recrystallizing deep-drawing steel



May 27, 19952 9 F. N. DARMARA RECRYSTALLI'ZING DEEPsDRAWING STEEL 2 SHEETS-SHEET l Filed Dec. v21, 1949 0. .a 0 7. 2 Z l .NN Q S Patented May 27, 1952 2,597,979 v REcRYsTALLIzING DEEP-DRAWING STEEL Falih N. Darmara, to United States of. New Jersey New Hartford, N, Y., assignor Steel fCompany, a corporation Application December 21, 1949, Serial No. 134,306 Claims. (Cl. 148-12) This invention relates to deep-drawing steels and more particularly to an improved heat treatment for developing an elongated grain structure therein.

Deep-drawing steels such as are used for forming automobile fenders and other articles requiring severe die forming operations are fully killed, 1owcarbon,lowmetalloid steels. These steels are commonly rolled to gauge by a heavy cold reduction of the order of 35% or more, following which they are box annealed at a temperature only slightly above their recrystallization temperature to recrystallize them and develop an elongated grain structure. If such steels are heated rapidly to a temperature above the recrystallization point, they will be equi-axed and this has heretofore precluded the use of continuous annealing which involves such rapid heating.

Such deep-drawing steels, with which this invention is concerned ordinarily contain between .03 and .15% carbon, .15 to .50% manganese, .04% maximum phosphorus, .04% maximum sulphur, excess amount of deoxidizing agent, preferably up to .08% maximum aluminum, with the balance iron except for other elements in residual amounts, although copper may be present up to .35% for special requirements.

I have discovered that if such steels are conditioned by heating them to temperatures somewhat less than their recrystallizing temperature, they can be rapidly heated to above the recrystallizing temperature without any harmful effect on the degree of elongation of the grains as compared to that obtained by box annealing.

Since there is no standard method of evaluating the degree of elongation of a recrystallized structure except by visual examination, which is inadequate, the following procedure was used in the test work involved in this invention. The average dimension of the grains is measured by drawing a 4" grid ruled at one inch intervals on the ground glass of a microscope. The field is projected at 500 on the glass and the number of grains crossed by the vertical and horizontal lines is counted. The former divided by the latter expresses the degree of elongation as the ratio of the average lengths of the grains in the rolling direction to that in the thickness direction, a ratio of at least 1.5 being desirable.

It is accordingly an object of this invention to provide an improved method of annealing coldreduced deep-drawing steel which will produce elongated grains therein.

It is a further object of the present invention u to eliminate the necessity reduced deep-drawing steels to develop elongated of box annealing coldgrains therein.

The foregoing and further objectswill-be Iape parent from the following description when read in conjunction with, the attached drawing, wherein:

Figure 1 is a graph illustrating the effects on nal grain shape by conditioning steel by my improved method wherein the ordinates .represent the elongation ratio and the abscissae conditioning time in minutes;

Figure 2 is a similar gure wherein the circled numbers represent the percentage of recrys. tallization during the conditioning treatment; and

Figure 3 is a series of graphs marked a, b, c,-d and e showing the influence timeand temperature of final recrystallization on` grain elongation' of deep-drawing steel conditioned at 1050 for one hour, wherein the numbers in circles represent the percentage of grains recrystallized.

As before stated, I have discovered that coldreduced deep-drawing steelcan be recrystallized in a short interval of time and anelongatedgrain obtained in the recrystallized structure if the steel is properly conditioned prior thereto. As shown by the foregoing gures, the conditioning must take place Within certain critical time and temperature limits. The experiments resulting in the data of the figures were conducted on a representative deep-drawing steel which had the following analysis:

C Mn P S Si Cu Ni Cr A1 A130:

and which had been cold reduced to gauge.

Figure 1 shows the effect of varying the time and temperature on the nal grain shape. In this iigure, line I shows the effect of heating the steel at 1000 F. for the varying times indicated; line 2, the effect of heating at 1050 F.; line 3, at 1100 F.; and line 4, at 1150 F. All samples were then recrystallized by heating to 1300 F. for two minutes. A review of the figure indicates the optimum temperature for conditioning is about 1050 F. and the effect thereof has dropped off materially by 1l00 F. However, at 1000 F. a material increase is noted.

In Figure 2, the percentage of recrystallization resulting from conditioning at various temperatures, line 5 shows the result of three-minute treatment; line 6, 10 minutes; and line l, 60

minutes. This again shows the marked effect in elongation obtained by treating at around 1050 F. and likewise shows that well over minutes and about 60 minutes is required for maximum results.

Figure 3 graphically illustrates the critical time and temperature conditions for recrystallizing steel conditioned as hereinbefore discussed. This iigure includes a series of graphs a, b. c, d, and e. In each of these, the ordinate indicates the ratio of grain size and the abscissa the recrystallization time in minutes. The encircled figures therein indicate the percentage of recrystallization at the times and temperature ishown. Graph a shows the eiects of recrystallizing at 1200 F. This temperature is not quite high enough to eiect complete recrystallization. The other graphs show that complete recrystallization is obtained in less than a minute at 1250, 1300, 1350 and 1400 F. However, at the latter temperature particularly and somewhat at 1350 F., the elongation ratio is lowered.

Thus, it is apparent that by preconditioning at a temperature between 1000 F. and 1100 F. and particularly at about 1050 F. for at least 10 minutes, but most eiectively at about an hour, deep-drawing steel can be completely recrystallized by quickly heating to recrystallizing temperatures. The recrystallizing is then practically instantaneous at 1350 F. and can be effected in less than a minute at temperatures of 1250 and 1300 F.

While I have shown and described several speclc embodiments of my invention, it will be understood that these embodiments are merely for the purpose of illustration and description and that various other forms may be devised within the scope of my invention, as defined in the appended claims.

I claim:

1. A method of producing an elongated grain structure in aluminum-killed deep-drawing steel comprising cold-reducing the steel at least 35% in thickness, conditioning said steel at a temperature between 1000 and 1100 F. for at least 10 minutes and then continuously annealing it at a temperature above its recrystallizing temperature.

2. A method of producing an elongated grain structure in aluminum-killed deep-drawing steel comprising cold-reducing the steel at least 35% in thickness, conditioning said steel at a temperature between 1000 and 1100 F. for at least 10 minutes and then recrystallizing it by continuously annealing it at a temperature between about 1200 and 14:00o F.

3. A method of producing an elongated grain structure in aluminum-killed deep-drawing steel comprising cold-reducing the steel at least 35% in thickness, conditioning said steel by holding it at a temperature of about 1050 F. for at least 10 minutes and then continuously annealing it at a temperature above its recrystallizing temperature.

4. A method of producing an elongated grain structure in aluminum-killed deep-drawing steel comprising cold-reducing the steel at least 35% in thickness, conditioning said steel by holding it at a temperature of about 1050 F. for about one hour and then recrystallizing it by continuously annealing it at a temperature between about 1200 and 1400 F.

5. A method of producing an elongated grain structure in aluminum-killed deep-drawing steel comprising cold-reducing the steel at least 35% in thickness, conditioning said steel by holding it at a temperature of about 1050 F. for about one hour and then recrystallizing it by continuously annealing it at a temperature between about 1250 and 1350 F.

'FALIH N. DARMARA.

REFERENCES CITED The following references are of record in the 

1. A METHOD OF PRODUCING AN ELONGATED GRAIN STRUCTURE IN ALUMINUM-KILLED DEEP-DRAWING STEEL COMPRISING COLD-REDUCING THE STEEL AT LEAST 35% IN THICKNESS, CONDITIONING SAID STEEL AT A TEMPERATURE BETWEEN 1000 AND 1100* F. FOR AT LEAST 10 MINUTES AND THEN CONTINUOUSLY ANNEALING IT AT A TEMPERATURE ABOVE ITS RECYSTALLIZING TEMPERATURE. 